1. Field of the Invention
The present invention generally relates to medical devices for filtering or removing matter from within a vascular system and the delivery of drugs to maintain continued filter patency. More specifically, the present invention relates to protective measures employing a drug delivery system for facilitating patency of the medical device. This device also relates to any other interventional applications where patency must be maintained. This includes such apparatus as stents, grafts, vessel liners and flow diverters.
2. Description of Related Art
Protective devices, generally, include an expandable filter attached to a hostwire. Protective devices are often employed in interventional cardiology/radiology applications to allow the flow of fluid, such as blood, while preventing the passage of particulate matter, such as emboli. Protective devices are often referred to as distal protection devices where the term ‘distal’ refers to the positioning of the device distal to a lesion or treatment site, with respect to fluid flow in a vessel. The filter portion of existing protection devices may include such items as braided meshes, woven fabrics, perforated films, a plurality of crossing wires, electrospun polymers and any other configuration suitable for filtering.
The performance of the protective device requires that the filter maintain patency. Patency is defined as the ability of the filter to allow the passage of fluid. Patency may refer to the patency of a filter at a specific point in time and/or the amount of time that a filter is able to maintain fluid flow. When used in a vascular system, the patency of the filter decreases over a period of time. As the pore size of the filter decreases, the patency will decrease relative to the patency for a greater pore size. For example, in some filters, when the maximum pore size is 100 um there may be pores ranging in size from 20 um or less. Such a fine pore size may cause a filter to become occluded by debris. Pores below a crucial pore size may also become occluded by formation of an impermeable fibrous sheet that may close off flow through the pore.
The current art utilizes three different mechanisms for facilitating patency.
The first mechanism used in the art involves the use of coatings on the filter to prevent blood clotting. Such coatings include anti-coagulants or anti-thrombogenic or anti-platelet or other such drugs, such as Heparin. Even with such coatings, the patency of the filter is limited because the drug coating is eventually overcome by clotting forces in the blood. Thus, this mechanism results in the patency beginning to decrease as soon as the coating contacts the clotting agents of the blood, and it is only a matter of time before filter patency is reduced or eliminated by the clotting agents.
Two other mechanisms used to provide for increased filter patency include dipping the filter in an anti-coagulant such as a Heparin solution, or a systemic use of drugs such as a IIb/IIIa inhibitor. Even with dipping in Heparin, the patency of the filter will deteriorate over a relatively short period of time. Problems with systemic use of drugs may manifest themselves as excessive patient bleeding.